In recent years, energy storage devices, especially lithium secondary batteries have been widely used as power supplies for electronic devices, such as mobile telephones, notebook-size personal computers and the like, power supplies for electric vehicles, as well as for electric power storage, etc. The batteries mounted on these electronic devices and vehicles may be used at midsummer high temperatures or in the environments warmed through heat generation by those electronic devices.
A lithium secondary battery, a type of energy storage device is mainly constituted of a positive electrode and a negative electrode containing a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution containing a lithium salt and a non-aqueous solvent. For the non-aqueous solvent, used are carbonates, such as ethylene carbonate (EC), propylene carbonate (PC), etc.
As the negative electrode of the lithium secondary battery, known are metal lithium, and metal compounds (metal elemental substances, oxides, alloys with lithium, etc.) and carbon materials capable of absorbing and releasing lithium. In particular, a lithium secondary battery using a nonaqueous electrolytic solution and using a carbon material capable of absorbing and releasing lithium, such as coke, graphite (artificial graphite, natural graphite) or the like, has been widely put into practical use. The above-mentioned negative electrode material stores and releases lithium and electron at an extremely electronegative potential equivalent to that for lithium metal, and therefore especially at high temperatures, there is a possibility that many solvents would be reductively decomposed by the negative electrode material of the type; and consequently, the solvent in the electrolytic solution would be partly reductively decomposed on the negative electrode irrespective of the type of the negative electrode material, and as a result, there occurs a problem in that lithium ion movement is thereby retarded owing to deposition of decomposed products and gas generation and the battery characteristics, such as cycle properties and the like especially at high temperatures are thereby worsened. Further, it is known that a lithium secondary battery using a lithium metal or its alloy, or a metal elemental substance, such as tin, silicon or the like or its metal oxide as the negative electrode material therein may have a high initial battery capacity but the battery capacity and the battery performance thereof, such as cycle properties greatly worsens, since the micronized powdering of the material is promoted during cycles thereby bringing about accelerated reductive decomposition of the non-aqueous solvent, as compared with the negative electrode of a carbon material.
On the other hand, a material capable of absorbing and releasing lithium, such as LiCoO2, LiMn2O4, LiNiO2 and LiFePO4 that are used as a positive electrode material stores and releases lithium and electron at a lithium-based electropositive voltage of not lower than 3.5 V, and therefore especially at high temperatures, there is a possibility that many solvents would be oxidatively decomposed by the positive electrode material of the type; and consequently, the solvent in the electrolytic solution would be partly oxidatively decomposed on the positive electrode irrespective of the type of the positive electrode material, and as a result, there occurs a problem in that lithium ion movement is thereby retarded owing to deposition of decomposed products and gas generation and the battery characteristics, such as cycle properties and the like are thereby worsened.
Despite the situation, electronic appliances equipped with lithium secondary batteries therein are offering more and more an increasing range of functions and are being in a stream of further increase in power consumption. With that, the capacity of lithium secondary batteries is being much increased, and the space volume for the nonaqueous electrolytic solution in the battery is decreased by increasing the density of the electrode and by reducing the useless space volume in the battery. Accordingly, the situation is that even decomposition of only a small amount of nonaqueous electrolytic solution may worsen battery performance at high temperatures.
PTL 1 proposes a nonaqueous electrolytic solution containing a phosphoric acid ester compound, such as triethylphosphonoacetate or the like, and indicates the possibility of enhancing continuous charging characteristics and high-temperature storage characteristics.